1. Field of the Invention
The present invention relates to an absolute angle detecting device, and more specifically, to the configuration of code pattern rows formed on a rotary disk and to the arrangement of detecting devices disposed opposite to the code pattern rows.
2. Description of the Related Art
An absolute angle detecting device is conventionally provided between a steering shaft and a body of an automobile. In general, the control of the damping force of a suspension, the control of the shift position of an automatic transmission, the steering control of rear wheels in a four-wheeled vehicle, etc., are all performed based on the steering angle, the steering speed, and the steering direction of a steering wheel detected by the absolute angle detecting device. In addition, the steering shaft is a multi-revolution body that makes two or three revolutions from a neutral position to the right or left direction. Therefore, an absolute angle detecting device is required for detecting the steering angle of the steering shaft.
As devices for detecting an absolute angle with a predetermined resolution, devices using a BCD code, devices using an M-system code, devices using a gray code, etc., have been used. Specifically, the devices using the gray code have been used more often. The reason is that, since the gray code is always changed by only one bit in each step, timing pulses are not required for reading, thereby achieving high resolution.
In an absolute angle detecting device for an automobile, in general, an angle (±720 degrees to ±1080 degrees) in which the steering shaft can move is divided into predetermined angles, which are called sectors, and a gray code is assigned with a predetermined resolution in each sector. The angle of one sector is determined according to the number of output bits or the accuracy of sector detection. However, one sector is generally set to 360 degrees or 180 degrees.
In addition, the absolute angle detecting device for an automobile controls security devices, such as a suspension and wheels. Therefore, it is necessary that the absolute angle detecting device have a high resolution of between 0.5 and 2 degrees in order to achieve high-precision control.
For this kind of absolute angle detecting device, the following devices have been suggested: a device in which one code pattern row is formed on a rotary disk, where nine detecting elements are arranged opposite to the code pattern row, and a resolution of 2 degrees is achieved; a device in which three code pattern rows are formed on a rotary disk, where the all of nine detecting elements are arranged opposite to the three code pattern rows, and a resolution of 1.125 degrees is achieved; a device in which two code pattern rows are formed on a rotary disk, where the all of ten detecting elements are arranged opposite to the two code pattern rows, and a resolution of 0.9 degrees is achieved. (for example, see Japanese Unexamined Patent Application Publication No. 2000-28396 (FIGS. 1 to 4). In all of the above devices there is one sector set to 360 degrees.
However, in the absolute angle detecting device for an automobile, it is strongly required that resolution have a value that can be easily divided, such as 1.5 degrees, 1.0 degrees, or 0.5 degrees, in order to easily handle signals detected by various control devices, in addition to achieving high resolution. Furthermore, the number of code pattern rows formed on a rotary disk and the number of detecting elements arranged opposite to the code pattern rows have an effect on the size and manufacturing costs of the absolute angle detecting device. Therefore, it is preferable that the number of code pattern rows and the number of detecting elements be small.
However, the conventional absolute angle detecting device does not meet the above-mentioned requirements. In addition, when one code pattern row is used as in the conventional technique, it is impossible to achieve a resolution of 1.5 degrees, 1.0 degrees, or 0.5 degrees in the detection of an absolute angle by 8-bit gray codes. Further, when one code pattern row is used as in the conventional technique, it is impossible to achieve a resolution of 1.5 degrees in the detection of an absolute angle by 9-bit gray codes.